贡献边际
参考答案：
指产品的销售收入与相应的变动成本之间差额，也称边际贡献、贡献毛益、边际利润。
下列离子方程式书写正确的是
A．铝粉投入到足量NaOH溶液中：2Al+2OH－═ 2AlO2－+H2↑
B．AlCl3溶液中加入足量氨水：Al3++ 3OH－ ═ Al(OH)3↓
C．碳酸氢钠溶液中加入足量澄清石灰水：HCO3-+OH-+Ca2+=CaCO3↓+H2O
D．Cl2与水反应：Cl2+H2O = 2H++Cl－+ClO－
Mr Wang often says to her child, "You _______ play with fire (火). It's dangerous.
[ ]
A. mustn't
B. had better
C. needn't
D. may not
